The lanthanide sesquioxides (Ln2O3) are of considerable commercial and military importance in high-powered laser applications. Typically they can act as hosts for activator ions in pumped solid-state lasers. In most solid-state lasers (as distinct from diode lasers) the device consists of a “pump” and an “emitter.” The emitter consists of the “host” and the “activator ion.” The pump can be a variety of energy sources and is typically a flash lamp, bright beam, or diode laser. These shine into the emitter to excite the activator ions to achieve a population inversion.
The activator ion is typically a metal ion that can absorb photons from the pump to achieve a population inversion. This inversion is created by a situation whereby an excited electronic state is more heavily populated than the energetic ground state. When this population inversion is achieved, the emitter “lases” or emits coherent radiation, namely a stream of photons that are of all the same wavelength and are in phase. Typically the activator ions are metal ions containing partially occupied d or f valence shells. Thus they can be, but are not limited to, Cr3+, Ti3+, Cr2+ (containing partially filled d valence electron shells) or many of the trivalent lanthanide ions containing partially filled f valence electron shells (often, but not limited to, Nd3+, Pr3+, Yb3+, Er3+, Tm3+, etc.). In a practical working solid-state laser, the concentration of activator ions in the host is fairly small, typically less than 1% by weight. The relatively low concentration is often preferred because of various deleterious relaxation events that can occur at higher concentrations.
The host has several requirements to perform well. It must readily accommodate the activator ion at the desired concentration (for example, 0.5-10% metal ion concentration) and generally form well in the preferred physical form (i.e., a large single crystal, a transparent ceramic slab, etc.). It typically must be electronically “innocent” in that it does not absorb pump photons or otherwise become involved in the electronic activity, absorption emission or other quantum mechanical events involved in lasing. It must have good ligand field properties so that the activator ion has good absorption cross-sections, appropriate lifetimes, suitable Stark levels and good emission behavior. It must have other physical and electronic properties that do not inhibit the performance of the activator ion (i.e. low thermal lensing, low birefringence, high optical damage threshold, etc.).
In modern lasers the typical pump source is a diode laser that pumps the host containing a suitable activator ion, wherein the host is a single crystal, a fiber or a transparent ceramic glass. As a general class these are typically called diode pumped solid-state lasers or DPSSLs. In high-powered lasers a typical design involves numerous diode lasers all simultaneously pumping a lasing emitter. Thus depending on power requirements more than 60 diode lasers can simultaneously pump one emitter.
The requirements for high-powered lasers (typically those with 1-100 kW power) include all of the traditional requirements as well as several other requirements that must be addressed for technological development. In particular there are several issues related to thermal management and thermal stability that are of extreme importance. All pumped solid-state lasers generate some amount of waste heat. This is because the pump typically excites the activator ion to an excited state that contains more energy than the emissive state. Thus the activator ion typically absorbs a photon from the pump to an excited state which then emits a small amount of energy before it lases, often relaxing from a higher energy Stark level to a lower energy Stark level. This slightly less energetic state is then the one that emits the coherent radiation or laser light. The small amount of energy that is emitted after excitation and before laser emission is typically given off as heat. For normal DPSSLs this excess heat is readily removed through a variety of standard water or air-cooling techniques. However, for lasers of extremely high power, i.e., greater than 100 W, the waste heat is often large enough to create a significant barrier to performance. The excess heat can cause decomposition of the host, phase changes or irregular thermal expansion that distorts the laser light (thermal tensing). Thus hosts for high-powered lasers must, in addition to the regular host requirements, have exceptional thermal stability and good thermal conductivity if they can be used in practical working devices. High thermal conductivity is a key requirement for good high powered laser hosts because the typical design envisions a slab or other shape that allows for substantial contact of the host with a heat drain such as a cooled metal. Good thermal conductivity allows for efficient waste heat removal from the host to the heat drain. The most common hosts for DPSSLs are usually metal oxides, which typically have relatively poor thermal conductivity. Thus the most desirable hosts for high-powered DPSSLs are the few oxides with outstanding thermal stability and decent thermal conductivity.
One class of new laser hosts that offer substantial advantages when implemented in high powered DPSSLs are the lanthanum sesquioxides Ln2O3 where Ln is typically chosen from the lanthanides or rare earths and commonly includes but is not limited to Sc, Y, La, Lu or Gd. These metal ions when in the form of their sesquioxides (Ln2O3) typically have very good ligand field properties as laser hosts. Thus they have fairly broad absorption bands, reasonable Stark level splittings, low phonon energies and long excited state lifetimes. These properties typically lead to desirable laser characteristics such as good cross sections and low thresholds. In addition the sesquioxides have several properties that give them improved performance as high-powered laser hosts. They have exceptional thermal stability, melting at extremely high temperatures (>2400° C.) and not undergoing any phase changes. Most importantly they have relatively high thermal conductivity. For example, Sc2O3 has a thermal conductivity about 50% greater than YAG (Y3Al5O12), which is one of the most common and standard DPSSL hosts in current practical devices. The use of the lanthanum sesquioxides as hosts is a considerable improvement because their high thermal conductivity enables the construction of high-powered DPSSLs where a slab or disc of the doped host can be placed on a metal cooling disc to drain the waste heat allowing for extreme pumping to very high powers.
There is, however, one significant drawback that prevents the implementation of the Sc2O3 and the other lanthanide sesquioxides as laser hosts in commercial devices. That drawback is the lack of high quality material available on a commercial or even experimental scale. While the oxides of appropriate formula Ln2O3 are readily commercially available, they are all in the form of powders with very small particle size. Such powders are worthless as hosts for DPSSLs. DPSSLs require a well-formed bulk sample of a size sufficient to align in a pump beam, can be coated with appropriate reflective coating and most importantly, have sufficient surface area to come in contact with a heat drain. However, manufacture of these materials in a form suitable for use in DPSSLs is extremely problematic. The forms most desirable for hosts in DPSSLs are high quality single crystals. In this case, the host is one single crystal of a size suitable for use (typically greater than 5 mm per edge) uniformly doped throughout with the appropriate concentration of the desired activator ion (Nd3+, Yb3+ or related). However, such single crystals are not easily obtained. The source of the difficulty lies in the extremely high melting point of these sesquioxides. Most of the lanthanide oxides Ln2O3 have melting points in excess of 2400° C. For example, Sc2O3 melts at 2460° C. and Y2O3 melts at 2430° C. These extreme melting points make growth of high quality single crystals suitable for DPSSLs very difficult. Typically high quality single crystals suitable for commercial optoelectronic applications are grown though a melt technique such as Czochralski pulling or Bridgeman growth. These all involve some form of melting and slow cooling of the pure oxide. Other related methods include top seeded solution growth or flux growth. These methods require a suitable flux that imparts reasonable solubility to the target compound. Other more exotic methods are sometimes employed such as laser pedestal heating or optical floating zone growth. These latter two are sometimes called containerless growth because, as their name implies, they do not require a container to hold the melt during growth.
These methods all have serious drawbacks for optimal growth and commercial production. The high melt temperature discourages the use of traditional Czochralski or Bridgeman methods because they require a high temperature crucible to grow. However, the only crucible material suitable to these extreme temperatures is Rhenium but at such high temperatures, the rhenium metal of the container becomes volatile and tends to impregnate the crystal during growth. These metal impurities lead to significant loss of performance as a DPSSL. Recently single crystals of Yb doped Sc2O3 were grown by the less common heat exchanger method and gave promising preliminary results as DPSSLs. However, this technique is somewhat exotic and not commonly used for commercial production of laser hosts. As discussed above, such growth of single crystals at extremely high temperatures typically leads to thermal strain, and thermally induced defects. During operation at high powers extreme heat is generated. Unfortunately under such extreme heat these defects lead to significant problems and the crystals perform very poorly making them not useful in high-powered lasers.
A more recent technology does not require the use of large single crystals of lanthanum oxides as laser hosts, but rather relies on the formation of transparent ceramics. In this technique, a suitably doped nanopowder of the host Ln2O3 is prepared by standard ceramic precursor methods and then heated and hot pressed under suitable vacuum conditions to form a clear solid ceramic slab. This technique has considerable promise in that it can lead to large (several centimeter) transparent slabs doped with the appropriate emitter ion at nearly any desired concentration. The performance of these materials is reasonable and comparable to that of the small single crystals prepared by the heat exchanger method described above. However, these materials are still ceramics that contain multiple grain boundaries. They are not single crystals so will not have the optimized ligand field parameters. Also the presence of large numbers of grain boundaries will limit the thermal conductivity of the solid in high-powered DPSSLs. There will also be applications where single crystal hosts will be necessary for optimized performance. Thus large single crystals of the lanthanide sesquioxides Ln2O3 are still the most desirable form of hosts for high-powered DPSSLs.
Hydrothermal crystal growth is one method to grow large single crystals of refractory oxides. It has been applied to a variety of oxides including quartz (SiO2), sapphire (Al2O3), potassium titanyl phosphate and several other materials. In this technique, a feedstock of powder or small crystals is placed in an autoclave or other high-pressure container. Typically one or more seed crystal is suspended above this feedstock. An aqueous solution is placed in the autoclave with a mineralizer dissolved therein. A mineralizer is typically a small ion like OH−, halide carbonate or H+ that induces a slight degree of solubility of the feedstock at high temperature. After the autoclave is sealed, it is heated to a temperature above the boiling point of water. Most commonly to induce large crystal growth, the autoclave is differentially heated such that the feedstock is in a hot zone and the seed crystal is in a cooler zone of temperature. Thus under suitable conditions, a supersaturation effect can be created so that feedstock is transported through the fluid and continuously deposited on the seed crystals to create a growing single crystal that can, in principal, grow as large as the autoclave or until the feedstock is exhausted. However, the formation of crystals by hydrothermal growth is highly dependant on the choice of mineralizer, concentration of mineralizer, growth temperature, thermal gradient, pressure, time and many other parameters. These vary widely for each particular crystal. Not all materials can be grown using this technique. This application has never been reported for the production of any lanthanide oxide crystals.
The older scientific literature contains several reports of the various lanthanide oxides exposed to hydrothermal conditions, namely water at the temperatures and pressures approximating those in this invention 300-700° C. at 5-20,000 pounds/square inch pressure. However, these reports were mainly concerned with the stability and phase relations of the metal oxides and their hydrated relatives. There is no report of large (greater than one millimeter) single crystal growth. Specifically, all the work in these papers describe powder, not crystal, formation.